Aqueous systems during film formation show increased sensitivity to foreign substances in the coating material or on the substrate which is to be coated, which sensitivity manifests itself in the form of craters, dents, or similar surface flaws. In primers, this sensitivity can in some cases be avoided or at least diminished by increasing the pigment content. Very frequently, however, particularly in the case of decorative coatings, this approach is not feasible since it leads to a reduction in the durability properties or to deterioration in the optical properties. Moreover, in electro-dip priming an increase in the pigment/binder ratio is possible only to a very limited extent for process engineering reasons.
A reduction in the sensitivity of aqueous coating systems to surface flaws can also be effected by addition of surface-active substances. In this case, an enrichment of the additives in the region of the coating surface occurs due to excessive incompatibility of the additives with the coating material which causes a substantial deterioration in the intercoat adhesion, that is to say the adhesion of further coats, in particular those based on vinyl chloride (co)polymers. As is known, PVC-based coatings are used on a large scale as underseals in the motor vehicle industry and are applied over the entire area of the cathodically deposited primer. To achieve the required stoneguard protection, the interlayer adhesion in these coating layers is of decisive importance.
To avoid extreme incompatibility, polymeric additives which are copolymers of acidic monomers and neutral ester monomers, mainly of (meth)acrylic monomers, are currently used for anionic coatings based on water-soluble paint binders carrying carboxylate groups. By appropriate matching of the acid fraction, optimum effectiveness can be achieved in these systems; but even in such systems there is high sensitivity to over-addition which immediately leads to the adhesion difficulties above described.
Austrian Patent Nos. 382,385; 383,137, and 382,630 disclose binders which can be deposited cathodically and which provide improved PVC adhesion due to their specific molecular structure. The avoidance of surface flaws is not observable in these products.
It is known to employ fluorine-containing monomeric compounds in the preparation of aqueous polymer dispersions which are primarily used for textile coating. Most of the fluorine-containing coating agents known from the literature are dispersions of tetrafluoroethylene polymers with special emulsifiers or special film-forming additives being employed. See, for example, JP 48/17545A; DE 2,837,107A; DE 3,403,880A; DE 3,533,807A; EP 173,071A, or EP 193,963. In EP-A2-0,293,963 reactive wetting agents containing fluorohydrocarbon radicals are described.